For many years, ultrasonic scanning systems have been utilized in medical and other applications to obtain images of body or machine portions not visible to the human eye. By use of well-known Doppler techniques, an indication of fluid flow, such as blood flow, within the scan area may also be obtained. More recently, proposals have been made to measure the power of the backscatter (i.e., the power of the echo signal in a direction 180.degree. to the direction of the incident ultrasonic signal) for various diagnostic purposes. For example, in an article entitled "Ultrasonic Characterization of Myocardium" in the September/October 1985 issue of Progress in Cardiovascular Disease, the quantitative relationship between the power of the backscatter of an ultrasonic signal and the condition of myocardium is discussed. In particular, the article discusses detecting myocardial ischemia (i.e., a lack of blood to the heart muscle) by viewing the absolute value of quantitative backscatter from the heart muscle. The absolute value of quantitative backscatter received from the heart muscle has a rhythmic variation of about 4 dB when the heart is normal. However, when the heart muscle is experiencing myocardial ischemia, the variations in power are blunted and, more important, the absolute value of the received power is perceptibly higher than when the heart muscle is normal. By detecting these changes in absolute value, it may be possible to quickly determine whether a patient has experienced or is experiencing a heart attack as well as its extent, something which may take several days of analysis using current techniques.
However, the above article does not suggest how quantitative backscatter may be determined. An article entitled "A Real-Time Integrated Backscatter Measurement System for Quantitative Cardiac Tissue Characterization", published in the January 1986 issue of I.E.E.E., Transactions on Ultrasonic Ferroelectrics and Frequency Control, l Vol. UFFC-33, No. 1, does describe an apparatus for obtaining quantitative backscatter signals. However, while the system described in this paper is capable of measuring backscatter power, it does not provide a true indication of the absolute value of quantitative backscatter, since it does not take into account attenuation of the transmitted signal from the ultrasonic transducer to the cardiac muscle or other point in the body at which the quantitative backscatter determination is to be made, and does not take into account the attenuation of the backscatter signal from this point. When used in this application, the term "point" shall be considered to be a small site, sample volume or resolution volume in the body. A point may, for example, appear as a single pixel in the display image of the scanned body area. While an estimated attenuation factor may be utilized to obtain a rough determination of quantitative backscatter, the accuracy of a quantitative backscatter determination made in this way may not be adequate for various diagnostic purposes such as, for example, for detecting myocardial ischemia, where the difference in absolute value for healthy and unhealthy tissue may not be great. For example, in the case of myocardial ischemia, the difference in absolute value of quantitative backscatter between a clearly healthy muscle tissue and one clearly undergoing myocardial ischemia, may be only 4 dB. A need therefore exists for a reliable method and apparatus for determining attenuation of backscatter signals, preferably in vivo, and for applying such determined attentuation to obtain the absolute value of quantitative backscatter for a given point in a subject's body.
Once the absolute value of quantitative backscatter has been determined, it is necessary that this information be displayed or otherwise provided to the physician or other health professional performing the procedure in a manner which will permit a determination as to the health of the patient to be quickly and accurately observed, and to quickly and accurately identify areas where potential problems exist. Methods and apparatus for displaying such information to achieve these objectives have not heretofore been known or available.